Beetlejuice
I've seen this error before on the rounded box code and I don't believe its got anything to do with the font.
Padding and margins are possible but if you haven't changed any sizes from the way x-cart does them in an original config then I'd suggest the problem isn't there either however it's worth checking them to make sure everything fits properly.
Since you have used code from another website then I'd suggest the problem might be similar to one mentioned above about extra or not needed <p> tags.
I've used this code on several occasions (both as is and edited) and in some cases I have found that there are tags with the coding listed in the first post that can cause errors on some sites.
Given that your site is showing an extra or a blank line I'd suggest your first thing to check would be tags like <p> or <br> or any other that produces white space. Just remove or comment out each line that you might suspect as the problem and if it doesn't work take out the comments and try the next line until you find the offending line.
If this doesn't work give us a web address so we can see the code as it is on your site.